I was under the impression that they didn't, because BA isn't listed on this page: https://www.westjet.com/en-ca/about-us/airline-partners

However, this article about the ending of the codeshare agreement between WestJet & BA says that the interline agreement would remain in place: https://www.straight.com/life/766976...nt-mid-october

I guess the fact that I'm able to purchase a single ticket from the BA website that includes a WestJet segment is proof enough that there is an interline agreement.